Benzyl reductive coupling

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

A crystal structure of the C02 derivative of (8), K[Co(salen)( 71-C02)], haso been reported in which the Co—C bond is 1.99 A, the C—O bonds are both equivalent at 1.22 A and the O-C-O angle is 132°.125 Carboxylation of benzylic and allylic chlorides with C02 in THF-HMPA was achieved with (8) electrogenerated by controlled-potential electrolysis,126 in addition to reductive coupling of methyl pyruvate, diethyl ketomalonate and / -tolylcarbodiimide via C—C bond formation. Methyl pyruvate is transformed into diastereomeric tartrates concomitant with oxidation to the divalent Co(salen) and a free-radical mechanism is proposed involving the homolytic cleavage of the Co—C bond. However, reaction with diphenylketene (DPK) suggests an alternative pathway for the reductive coupling of C02-like compounds. 

In addition to the reductive coupling reaction of ketones, certain alcohols can also be reductively coupled using active uranium. Benzhydrol is coupled by active uranium to give TPA as the only coupled product. No TPE is seen. Under similar conditions, no coupling of benzyl alcohol is seen. The chemistry of the active uranium is under continued investigation. 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Complexes of N-N bonded dinitrogen dioxide, such as depicted in pathway B of Scheme 5, would appear to be necessary in order to effect the formation of the N-N bond. This has been treated theoretically as a metal promoted reductive coupling of 2 NO to form a hyponitrite complex (79). The Cu Tp112) system was also shown to catalyze NO oxidations of benzyl and isopropyl alcohol to benzaldehyde and acetone (Eq. (37)). Electrospray mass spectrometry indicated that higher... 

Lund and coworkers [131] pioneered the use of aromatic anion radicals as mediators in a study of the catalytic reduction of bromobenzene by the electrogenerated anion radical of chrysene. Other early investigations involved the catalytic reduction of 1-bromo- and 1-chlorobutane by the anion radicals of trans-stilhene and anthracene [132], of 1-chlorohexane and 6-chloro-l-hexene by the naphthalene anion radical [133], and of 1-chlorooctane by the phenanthrene anion radical [134]. Simonet and coworkers [135] pointed out that a catalytically formed alkyl radical can react with an aromatic anion radical to form an alkylated aromatic hydrocarbon. Additional, comparatively recent work has centered on electron transfer between aromatic anion radicals and l,2-dichloro-l,2-diphenylethane [136], on reductive coupling of tert-butyl bromide with azobenzene, quinoxaline, and anthracene [137], and on the reactions of aromatic anion radicals with substituted benzyl chlorides [138], with... 

A mixture of isomers was, however, obtained if the a,a -dibromoxylene was not symmetrical. This difficulty was overcome by using a reductive coupling of a substituted 3,4-dihydroisoquinoline (71) as its salt with an o-methoxycarbonyl-substituted benzyl bromide (73)129 [Eq. (60)]. 

Reductive coupling benzyl ketones.1 Benzyl bromides and acyl chlorides undergo reductive coupling when treated with zinc (2 equiv.) and catalytic amounts of Pd[P(Cr,H5)3]4 or Cl2Pd[P(C6H5)3]2 to form benzyl ketones. The highest yields are obtained with aryl acyl chlorides. ArCHzZnBr is considered to be an intermediate. 

Amino acids can be synthesized via the coupling of Schiff bases with carbanions generated by reduction of benzyl halides [143,144]. Electrolysis of benzyl chloride at mercury in DMF containing TBABr, and in the presence of the Schiff base formed from benzylamine and benzyl pyruvate, affords or-methylphenylalanine in up to 86% yield (after hydrogenolysis over palladium-charcoal). An antihypertensive agent, or-methyl-j6-(3,4-dihydroxyphenyl)alanine, has been prepared by reductive coupling of the aforementioned Schiff base with 3,4-methylenedioxybenzyl chloride. 

Coupling of allylic and benzylic alcohols (2, 415). This 3 1 reagent (or the commercial mix) converts allylic and benzylic alcohols into the products of reductive coupling. The reaction is carried out in refluxing glyme (16 hr.). Products of primary-primary and primary-tertiary coupling are obtained from famesol (last example). 

Fluoro- and chloro-substituted tricarbonyl chromium arene complexes can be reductively coupled with ketones, in the presence of samarium iodide, to give tertiary alcohols (Scheme 109). A related reaction of a tethered alkoxyimine furnished a tricyclic compound (Scheme 110). Samarium also promotes the formation of a benzylic ketyl... 

The chromium-complexed benzylic radicals are also known and were employed in a variety of useful organic reactions. One electron reductive coupling of the tricarbonylchromium complexes of o-substituted benzaldehydes and benzaldi-mines with samarium iodide gave 1,2-diols and diamines, respectively, in good yields with extremely high threo diastereoselectivity (Eq. 7) [7]. Therefore, enantiomerically pure 1,2-diols and diamines were easily obtained by using the planar chiral chromium complexes of benzaldehyde and benzaldimine. 

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